This invention relates in general to polyimide foam structures and, more specifically, to methods of preparing polyimide foam structures having complex shapes and unique characteristics.
Polyimide resins, as coatings, adhesives, foams and the like have come into widespread use due to their chemical inertness, strength, high temperature resistance and flame resistance. Polyimide foam in the form of sheets and panels are often used as thermal insulation in high temperature environments. Typical polyimide foams include those described in U.S. Pat. Nos. 4,425,441 (Gagliani et al), 4,426,463 (Gagliani et al), 4,518,717 (Long et al), 4,562,112 (Lee et al), 4,621,015 (Long et al) and 4,647,597 (Shulman et al).
Sheets and panels of foam are generally made by causing a layer of liquid or powder precursor on a flat surface to foam without restriction, then slicing the foam at a desired thickness parallel to the surface, to remove the rind that forms on the free surface. This generally produces a foam sheet of optimum uniformity and low density. These sheets may then be adhesively bonded to face sheets to form walls, insulating panels or the like. While useful in many applications, these panels may not have sufficient strength and stiffness for some applications and only very simple structures may be made by this method.
The liquid or powder polyimide foam precursor may also be heated to the foaming and curing temperatures in a closed mold coated with a mold release to form a sheet or other desired shaped foam product as described, for example, by Gagliani et al in U.S. Pat. No. 4,425,441. While a variety of shapes may be formed, the restricted foam expansion often produces foams of uneven density and higher overall density than is generally desired. Problems with closed mold foaming also include variations in cell size, often with large voids, and heavy skins at the tool surfaces. Thus, it is difficult to produce foam products with higher density and strength than those produced by the free foam and trim process.
With complex shapes, it is often impossible to distribute the precursor in powder liquid form in a manner which will produce uniform density in adjacent thin, thick or undercut regions. Also the final structure may not have sufficient strength and stiffness for some purposes where density and strength varies because of the inability to properly distribute the starting materials in a complex mold.
In some cases, as described by Hill in U.S. Pat. No. 4,874,648, a block of uniform density polyimide foam can be reshaped by compressing selected areas to produce a more complex shape. However, the density of the product will vary, with higher density in the compressed region and very complex, undercut, products are very difficult to produce.
For some purposes, it is desirable to distribute other materials throughout a polyimide foam shape in either a uniform or non-uniform distribution. Where the material to be distributed has a higher density, such as metal, ferrite or carbon particles, the material will tend to settle or be otherwise disturbed during foaming, even if evenly distributed in the precursor. Varying the distribution of the additive particles in any specific desired manner is generally not possible with any accuracy.
Thus, there is a continuing need for improved methods of producing complex shapes in polyimide foam materials, especially where uniform density is required or particulate material is to be distributed through the foam in a selected pattern.